Broadcast and multicast are methods for transmitting datagrams from a single source to several destinations, i.e. point-to-multipoint connection. The 3GPP (3rd Generation Partnership Project) specifications Release-4 and Release-99 define a cell broadcast service (CBS) allowing for low bit-rate data to be transmitted as a message-based service to all subscribers in a set of given cells over a shared broadcast channel. Furthermore, an IP-Multicast service is defined allowing for mobile subscribers to receive multicast traffic. However, this service does not allow for multiple subscribers to share radio or core network resources and as such does not offer any advantages as far as resource utilization is concerned within a PLMN (Public Land Mobile Network) and over a RAN.
In general a broadcast multicast service is a unidirectional point-to-multipoint service in which data is efficiently transmitted from a single source to multiple terminal devices or user equipments (UE) in the associated broadcast service area. Cell Broadcast services may be received by all users who have enabled the specific broadcast service locally on their UE and who are in the cell broadcast area defined for the service. In contrast thereto, multimedia broadcast multicast i.e., Multimedia Broadcast/Multicast Service (MBMS) services can only be received by such users that are subscribed to a specific multicast/broadcast service and in addition in a multicast mode have joined the multicast group associated with the specific service. Multicast subscription may he performed either upon user selection or due to home environment initiation.
It is noted that the abbreviation “UE” in this specification refers to both mobile terminal or mobile station (MS) in GSM (Global System for Mobile communications) terms and user equipment in UMTS (Universal Mobile Telecommunications System) terms.
It is envisaged that for some applications, multiple users can receive the same data at the same time. The benefit of multicast and broadcast in the network is that the data is sent once on each link. For example, in case of a GPRS (General Packet Radio Services) based core network, a Serving GPRS Support Node (SGSN) will send data once to a Radio Network Controller (RNC) of the RAN regardless of the number of base station devices, e.g. Node Bs in UMTS terms, and to UEs that wish to receive it. The benefit of multicast and broadcast on the air interface is that many users can receive the same data on a common channel, thus not clogging up the air interface with multiple transmissions of the same data.
With increasing use of high bandwidth applications in 3rd generation mobile systems, especially with a large number of users receiving the same high data rate services, efficient information distribution is essential. Thus, broadcast and multicast are techniques to decrease the amount of data within the network and use resources more efficiently.
Point-to-multipoint services are expected to be used extensively over wireless networks, so that there is need for a capability in the PLMN to efficiently support them. In the 3GPP specifications TS 22.146 and TR 23.846, a MBMS is defined to provide this capability for broadcast/multicast services offered by the home environment and other value added service providers. The MBMS is a unidirectional point-to-multipoint bearer services in which data is transmitted from a single source entity to multiple recipients. In particular, a broadcast mode and a multicast mode is defined as nodes of operation of the MBMS.
An assumption made in the above 3GPP specifications for MBMS defines that for each MBMS service, the respective Control RNC (CRNC) or Serving RNC (SRNC) should have an MBMS context. In practice, this means that service contexts are configured at the RNC, which are not assigned to any specific UE, whereas this context is used by a number of UEs in the cell. In order to link this context with the UE specific active set e.g. of RRC (Radio Resource Control), i.e. the set which describes all the connections assigned for the UE in question, it is required that the RNC should detect those UEs which are requesting the MBMS service and for which MBMS services the RNC has already generated corresponding MBMS contexts. The RRC is a sublayer of the radio interface Layer 3 existing in the control plane only, which provides information transfer service to the non-access stratum, e.g., the core network. RRC is responsible for controlling the configuration of radio interface Layers 1 and 2 according to the OSI (Open System Interconnection) protocol layer architecture.
According to the 3GPP specification TS 25.331, the state of a UE can be divided between two ruling states RRC connected state or mode and RRC idle state or mode. In addition thereto, four sub-states have been defined. These sub-states are Cell_DCH, Cell_FACH, Cell_PCH and URA_PCH. For each of these sub states the above specification defines the transactions, which can be supported and the rules when the switching between the RRC connected states is allowed to be made.
After power on the UE enters into an Idle mode, upon which the first task to perform is the so-called IMSI/GPRS Attach procedure. As a result of this procedure the MM (Mobility Management) context is established for the UE into the core network (CN) side, which allows the activation of the MM functions for the UE. All signaling, which is required to exchange between the CN and the UE upon this phase, is made in the RRC connected mode, from which it is returned back to the Idle mode if no other procedures (e.g. mobile originated call) has been initialized after successful attachment procedure.
The UE stays in Idle Mode until it transmits a new request to establish an RRC Connection to the network. In Idle Mode the connection of the UE is closed on all layers of the access stratum. In Idle Mode the UE is identified by non-access stratum identities. In addition, the RAN has no own information about the individual Idle Mode UEs, and it can only address e.g. all UEs in a cell or all UEs monitoring a paging occasion. The RRC Connected Mode is entered when the RRC Connection is established.
The RRC states within the RRC Connected Mode reflect the level of UE connection and which transport channels that can be used by the UE. The transition to the RRC Connected Mode from the Idle Mode can only be initiated by the UE by transmitting a request for an RRC Connection. The event is triggered either by a paging request from the network or by a request from upper layers in the UE. When the UE receives a message from the network that confirms the RRC connection establishment, the UE enters the CELL_FACH or CELL_DCH state of RRC Connected Mode.
In the case of a failure to establish the RRC Connection the UE goes back to Idle Mode. Possible causes are radio link failure, a received reject response from the network or lack of response from the network (timeout).
In the CELL_DCH state, a dedicated physical channel is allocated to the UE in uplink and downlink, and its location is known on cell level based on UE mobility functionality activated in RRC connected mode. Dedicated transport channels, downlink and uplink, shared downlink transport channels, and a combination of these transport channels can be used by the UE. The CELL_DCH-state is entered from the Idle Mode through Cell-FACH state by initializing of the establishment of an RRC connection, or by sending the Cell Update request as a response to the received paging message.
In the CELL_FACH state, no dedicated physical channel is assigned to the UE. The UE continuously monitors an FACH in the downlink. The UE is assigned default common transport channels, i.e. Fast Access Channel (FACH) in downlink and Random Access Channel (RACH) in uplink, that it can use anytime according to the access procedure for that transport channel. The position of the UE is known by the RAN on cell level according to the cell where the UE last made a cell update. Upon release of the RRC connection, the UE moves to the idle mode.
In the CELL_PCH state, no dedicated physical channel is allocated to the UE and the UE is not allowed to send any data through common channels. The UE selects a PCH with a specific algorithm, and uses DRX (Discontinuous Reception) for monitoring the selected PCH. No uplink activity is possible. The position of the UE is known by the RAN on cell level according to the cell where the UE last made a cell update in CELL_FACH state. The DCCH (Dedicated Control Channel) logical channel cannot be used in this state. If the network wants to initiate any activity, it needs to make a paging request on the PCCH (Paging Control Channel) logical channel in the known cell to initiate any downlink activity. The UE is transferred to CELL_FACH state by paging from the RAN or through any uplink access.
In the URA_PCH state no dedicated channel is allocated to the UE. The UE selects a PCH with a specific algorithm, and uses DRX for monitoring the selected PCH via an associated PICH. No uplink activity is possible. The location of the UE is known on registration area level according to the registration area assigned to the UE during the last area update in CELL_FACH state. The DCCH logical channel cannot be used in this state. If the network wants to initiate any activity, it needs to make a paging request on the PCCH logical channel within the registration area where the location of the UE is known. If the UE needs to transmit anything to the network, it goes to the CELL_FACH state.
The current system is defined for point-to-point connections. However, the MBMS system cannot be designed in such a way that all authorized MBMS UEs are sending the responses to the network almost simultaneously caused by e.g. MBMS notification, to switch to the CELL-DCH state, which is required especially when the data is decided to sent by using the point-to-point connections.